How can I sprintf a big number in Perl?

Question

On a Windows 32-bit platform I have to read some numbers that, this was unexpected, can have values as big as 99,999,999,999, but no more. Trying to sprintf("%011d", $myNum) them outputs an overflow: -2147483648.

I cannot use the BigInt module because in this case I should deeply change the code. I cannot manage the format as string, sprintf("%011s", $numero), because the minus sign is incorrectly handled.

How can I manage this? Could pack/unpack be of some help?

Answer 1

Try formatting it as a float with no fraction part:

$ perl -v
This is perl, v5.6.1 built for sun4-solaris
...

$ perl -e 'printf "%011d\n", 99999999999'
-0000000001

$ perl -e 'printf "%011.0f\n", 99999999999'
99999999999

Answer 2

Yes, one of Perl's numeric blind spots is formatting; Perl automatically handles representing numbers as integers or floats pretty well, but then coerces them into one or the other when the printf numeric formats are used, even when that isn't appropriate. And printf doesn't really handle BigInts at all (except by treating them as strings and converting that to a number, with loss of precision).

Using %s instead of %d with any number you aren't sure will be in an appropriate range is a good workaround, except as you note for negative numbers. To handle those, you are going to have to write some Perl code.

Answer 3

I'm no Perl expert, and maybe I'm missing some sort of automatic handling of bignums here, but isn't this simply a case of integer overflow? A 32-bit integer can't hold numbers that are as big as 99,999,999,999.

Anyway, I get the same result with Perl v5.8.8 on my 32-bit Linux machine, and it seems that printf with "%d" doesn't handle larger numbers.

Answer 4

I think your copy of Perl must be broken, this is from CygWin's version (5.10):

pax$ perl -e 'printf("%011d\n", 99999999999);'
99999999999

pax$ perl -v

This is perl, v5.10.0 built for cygwin-thread-multi-64int
(with 6 registered patches, see perl -V for more detail)

Copyright 1987-2007, Larry Wall

Perl may be copied only under the terms of either the Artistic License or the
GNU General Public License, which may be found in the Perl 5 source kit.

Complete documentation for Perl, including FAQ lists, should be found on
this system using "man perl" or "perldoc perl".  If you have access to the
Internet, point your browser at http://www.perl.org/, the Perl Home Page.

What version are you running (output of perl -v)?

You may have to get a 64-bit enabled version of Perl [and possibly a new 64-bit production machine] (note the "cygwin-thread-multi-64int" in my output). That will at least avoid the need for changing the code.

I'm stating this on the basis that you don't want to change the code greatly (i.e., you fear breaking things). The solution of new hardware, whilst a little expensive, will almost certainly not require you to change the software at all. It depends on your priorities.

Another possibility is that Perl itself may be storing the number correctly but just displaying it wrong due to a printf() foible. In that case, you may want to try:

$million = 1000000;
$bignum = 99999999999;
$firstbit = int($bignum / $million);
$secondbit = $bignum - $firstbit * million;
printf ("%d%06d\n",$firstbit,$secondbit);

Put that in a function and call the function to return a string, such as:

sub big_honkin_number($) {
    $million = 1_000_000;
    $bignum = shift;
    $firstbit = int($bignum / $million);
    $secondbit = $bignum - $firstbit * $million;
    return sprintf("%d%06d\n", $firstbit, $secondbit);
}
printf ("%s", big_honkin_number (99_999_999_999));

Note that I tested this but on the 64-bit platform - you'll need to do your own test on 32-bit but you can use whatever scaling factor you want (including more than two segments if need be).

Update: That big_honkin_number() trick works fine on a 32-bit Perl so it looks like it is just the printf() functions that are stuffing you up:

pax@pax-desktop:~$ perl -v

This is perl, v5.8.8 built for i486-linux-gnu-thread-multi

Copyright 1987-2006, Larry Wall

Perl may be copied only under the terms of either the Artistic License or the
GNU General Public License, which may be found in the Perl 5 source kit.

Complete documentation for Perl, including FAQ lists, should be found on
this system using "man perl" or "perldoc perl".  If you have access to the
Internet, point your browser at http://www.perl.org/, the Perl Home Page.

pax@pax-desktop:~$ perl qq.pl
99999999999